实现垂直居中

1.table自带功能

<table class="parent">     
    <tr>
        <td>
        ...........
        ...........
        ...........
        </td>
    </tr>
</table>
.parent{
height:600px;
}

2.将div变成table

<div class="table">
    <div class="td">
        <div class="child">
        .............
        .............
        .............
        </div>
    </div>
</div>
div.table{
    display:table;
    height:600px;
}
div.td{
    display:table-cell;
    vertical-align:middle;
}

3.margin-top:-50%

<div class="parent">
    <div class="child">
      ..........
    </div>
.parent{
  height: 600px;
  border: 1px solid red;
  position: relative;
}
.child{
  border: 1px solid green;
  width: 300px;
  position: absolute;
  top: 50%;
  left: 50%;
  margin-left: -150px;
  height: 100px;
  margin-top: -50px;
}

4.flex

<div class="parent">
    <div class="child">
      ...........
    </div>   
 </div>
 .parent{
  height: 600px;
  border: 3px solid red;
  display: flex;
  justify-content: center;
  align-items: center;
}
.child{
  border: 3px solid green;
  width: 300px;
}